1. Field of the Invention
The present invention relates to an AM radio receiver apparatus of a digital tuning system employing a phase locked loop (PLL).
2. Description of the Related Art
A superheterodyne radio receiver apparatus is arranged as follows.
Specifically, a broadcasting wave is received by an antenna to obtain a weak signal. The weak signal is amplified by a high-frequency amplifier. A mixer circuit mixes the amplified signal with a local oscillation signal to obtain an intermediate-frequency signal. Since an output signal from the mixer circuit contains a signal component other than the intermediate-frequency signal, an intermediate-frequency selecting device (filter) selects only the intermediate-frequency signal from the output signal from the mixer circuit. An intermediate-frequency amplifier amplifies the intermediate frequency signal. An AM detecting circuit detects the amplified intermediate-frequency signal to obtain an audio signal component.
Since a difference between a reception frequency and a frequency of a local oscillation signal is an intermediate frequency in this case, a reception frequency is determined by changing a value of the local oscillation frequency. This operation of changing the value of the reception frequency is called the tuning.
A tuning system of a radio receiver apparatus includes an analog tuning system and a digital tuning system. The analog tuning system employs a variable capacitor. When a tuning dial is rotated, a local oscillation frequency is continuously changed, thereby a desired reception frequency being selected.
The digital tuning system has a local oscillator circuit formed of a phase locked loop (PLL). When a frequency dividing ratio N of a variable frequency divider forming a part of the PLL is changed, in case of an AM radio receiver, for example, a local oscillation frequency is changed stepwise at an interval of 9 kHz or 10 KHz to thereby change a reception frequency, whereby a desired broadcasting frequency is selected. The above frequency dividing ratio N is given by a microcomputer, for example.
When the digital tuning system is employed, if the frequency dividing ratio N used for obtaining the local oscillation frequency used when the mixer circuit converts a reception frequency of a desired broadcasting station into an intermediate frequency is previously set in a memory so as to correspond to a button of a radio receiver, then it is possible to select the broadcasting of the desired reception frequency only by pressing the button. Since a fine adjustment carried out by using the tuning dial in the analog tuning system is not necessary, this facilitate a user's operation. Moreover, a radio receiver employing the digital tuning system is easily formed of ICs.
Therefore, most of recent radio receivers employ the digital tuning system employing the PLL rather than the analog tuning system.
In the digital tuning system, the intermediate frequency must have a fixed value. However, if a selection characteristic of the intermediate-frequency selecting device (filter) is fluctuated (i.e., a center frequency of a pass bandwidth of a filter is fluctuated), then the intermediate frequency is fluctuated. Moreover, since the difference between the reception frequency and the local oscillation frequency is the intermediate frequency, the fluctuation of the intermediate frequency leads to fluctuation of the reception frequency.
When the radio receiver employs the analog tuning system, the local oscillation frequency can be continuously changed as described above. Therefore, even if the reception frequency is fluctuated by several kHz due to the fluctuation of the intermediate frequency, then it is sufficient to finely adjust the reception frequency by using the tuning dial.
However, when the radio receiver employs the digital tuning system, the reception frequency is changed stepwise at an interval of 9 kHz or 10 kHz as described above. Therefore, if the reception frequency is fluctuated by several KHz, then the broadcasting of the desired broadcasting station may be prevented from being received.
Specifically, if the intermediate-frequency selecting device is formed of an active filter formed of a capacitor and a resistor in an IC, then when a resistance value is fluctuated by about 20%, a frequency of a pass bandwidth of the active filter formed in the IC is also fluctuated to that extent.
If the intermediate frequency is selected to be a frequency smaller than 100 kHz, e.g., 50 kHz for comparatively reducing the number of steps of the filter and the number of elements thereof, then when a center frequency of a pass bandwidth of the intermediate-frequency selecting device (filter) is fluctuated by about 10 KHz, the broadcasting of the desired broadcasting station cannot be received. In general, when an IC is manufactured normally, a resistance element of the IC is fluctuated by .+-.20% within a dispersion error range. Therefore, in practice, upon a circuit design of an IC, the values are previously set on the assumption of the dispersion error of about 20%.
In order to avoid such disadvantageous fluctuation of the intermediate frequency, the conventional radio receiver of the digital tuning system has employed a ceramic filter as a filter for selecting an intermediate frequency.
However, the ceramic filter must be attached to the IC from the IC, which increases the number of connection terminals of the IC and prevents the IC from being downsized. Moreover, the ceramic filter is expensive and is a large-size part, which substantially prevents the radio receiver from being made thinner and smaller.